The alkaline leaching of vanadium-bearing materials is a known process for the recovery of vanadium values from carbonaceous residues derived from petroleum refining or from the burning of carbonaceous fuels. Vanadium and other metals, being relatively non-volatile, remain in the residues during refining and in the ash during combustion as fuels, as from example, in power plants. However, the efficient and economic extraction of vanadium from these carbonaceous residues without the simultaneous extraction of deleterious amounts of priority pollutant metals, such as lead, cadmium, and chromium, has remained a problem.
Although sulfuric acid has been used successfully as a leachant for certain residues, such as fly ash, acid leaching is also known to dissolve appreciable quantities of toxic metals present. Moreover, many power plant residues, for example, boiler ash and scrubber sludges, have been treated with bases, such as lime, limestone, or magnesium hydroxide to prevent corrosion and to scrub acidic components released during combustion. The presence of these bases can require extremely high consumption of acid during attempts to extract vanadium by acid leaching. Hence, for economic considerations acid leaching of such residues is usually not warranted.
It is also known that alkaline leaching can be used to extract vanadium from base-containing residues provided that vanadium is in an oxidized state (pentavalent) or that an oxidant is present during the leaching procedure. Although alkaline leaching generally represses the dissolution of priority pollutant metals, vanadium recoveries are frequently lower. Because alkaline leaching extracts less metals, the further processing of the liquor is simplified, and the effluents can be readily acceptable for disposal.
Various methods have been proposed in the literature for the recovery of vanadium values by leaching with alkaline materials. For example, in U.S. Pat. No. 3,873,669, there is disclosed a process for the recovery of vanadium from fly ash by treating the fly ash with a concentrated caustic soda solution to selectively solubilize vanadium, and then adding calcium oxide to precipitate silica before recovering the vanadium.
A further method has been disclosed in the patent literature for the recovery of vanadium values by leaching the vanadium-containing material with caustic and thereafter precipitating insoluble salts of vanadium with an ammonium containing compound such as gaseous ammonia or ammonium hydroxide. This method is disclosed in U.S. Pat. No. 4,061,712 and is indicated to provide greater yields of the desired product.
As indicated above, vanadium-bearing residues which contain appreciable amounts of alkaline earth bases such as MgO, CaO, CaCO.sub.3, MgCO.sub.3, Ca(OH).sub.2, Mg(OH).sub.2 are generally quite refractory to alkaline leaching. The vanadium leach yields are often less than 50 percent which is not satisfactory. Usually leaching with Na.sub.2 CO.sub.3 gives inferior results to leaching with NaOH.
The object of the alkaline leach is, of course, to form highly water-soluble vanadates, e.g., sodium vanadates such as NaVO.sub.3, Na.sub.4 V.sub.2 O.sub.7, or Na.sub.3 VO.sub.4. This can be conveniently represented by the equation: EQU V.sub.2 O.sub.5 (s)+2NaOH=2NaVO.sub.3 +H.sub.2 O
where NaVO.sub.3 (sodium metavanadate) has a high solubility, i.e., greater than 100 grams V.sub.2 O.sub.5 /liter.
Normally, V.sub.2 O.sub.5 is not the actual species present in these residues. Various calcium and magnesium vanadates are present which dissolve with much greater difficulty. Also, some vanadates may be reduced, which requires an oxidant along with an alkaline leach reagent to solubilize the vanadium. Oxygen, air or hydrogen peroxide can serve as oxidants. Other more expensive but less practical oxidants could be used also, such as ammonium persulfate, potassium permanganate, and sodium hypochlorite.
Accordingly, prior to the present invention there had been no simple and efficient method for the recovery of vanadium from residues in yields higher than many of the methods currently employed in commercial operations.